With the increasing diminution of the structures to be reproduced, electron-beam exposure procedures, which include X-ray beam, electron-beam and ion-beam procedures, are increasingly replacing optical lithography. They are used for manufacturing exposure masks or for the direct exposure of semiconductor wafers and in the case of the structure widths aspired to of only approx. 0.2 .mu.m in the case of the 1 GBit chip generation they will be the dominant lithography processes.
While the mask substrates for conventional optical lithography consist of relatively thick (several mm) quartz plates, which are transparent for the normal light wavelengths, it has been attempted since as the beginning of the 1970s to use membrane masks for X-ray beam, electron-beam and ion-beam procedures. This permits an adequate throughput of semiconductor wafers at a higher resolution. The interactions of these three types of short-wave radiation with the mask call for membrane masks with a thickness of roughly 0.1 .mu.m up to several .mu.m.
The masks for ion-beams require holes in the membrane as a pattern, while for X-ray beam and electron-beam exposure closed membranes with metal-absorber patterns can also be used.
In all these cases the membrane mask is manufactured by an electron-beam pattern generator writing the appropriate patterns in photoresist. For structures smaller than 0.5 .mu.m, the corner quality of the patterns written using electron beams is poor and the corners are rounded.
The pattern is then transferred to the membrane or the absorber layer by etching processes. The anisotropic plasma-etching processes mostly used are distinguished by exact pattern transfer, i.e. the corners which are already rounded in the photoresist are transferred to the membrane as rounded corners of almost identical dimensions.
Shadow masks, or hole masks, as shown in FIG. 4, in which the is pattern consists of physical holes and which are described for example in European patents EP 0 019 779 or EP 0 078 336, have been realized hitherto exclusively using a membrane 10 of silicon.
In the case of the shadow or stencil mask in EP 0 019 779, the n-doped silicon substrate has a p-doped surface layer, the membrane, which is coated with a thin chromium layer and two gold layers applied over it. This gold layer, which is typically several hundred nm (approx. 1 .mu.m maximum) thick in total, was used to stop the electrons in the impermeable mask areas.
The membrane thicknesses are in the region of roughly 1 to 4 .mu.m, typically 2 .mu.m. Silicon membranes of this kind can be manufactured uniformly with the use of a doping etch stop of boron. As structure sizes become smaller and membrane thickness decreases, the demands on anisotropic plasma etching become ever greater and an extremely high p-doping is required as a doping etch stop, e.g. boron doping of approx. 1.3.times.10.sup.20 boron atoms/cm.sup.3. Silicon membranes with this degree of etch-stop doping display a high number of misalignment faults and mechanically are extremely fragile.
In masks with a closed oro continuous membrane for X-ray beam lithography, as shown in FIG. 5, the pattern is produced in the form of a structured metal absorber material 21 on the continuous membrane 20. in order for the membrane to be transparent to X-ray beams, it should be only a few pm thick and the membrane material should have an atomic number that is as small as possible, in order to absorb as little radiation as possible at the "transparent" areas.
The absorber material is also only a few pm thick and has as high an atomic number as possible. Typical metal absorbers consist of tungsten or gold and for membrane materials such as silicon, silicon nitride, silicon carbide, a silicide as proposed in EP 0 048 291 or recently also diamond have been selected.
The base for the membrane is a silicon wafer 22, which due to anisotropic etching has at least one opening going right through, the side walls of which consist of (111) planes and are inclined at 54.7.degree. to the (100) surface of the silicon wafer.
With these masks, the problem of mask distortions caused by uneven mechanical stress in the membrane has not been solved satisfactorily to date. Mechanical distortions can be caused both by the membrane material itself and by the absorber material. In addition, the difficulty exists of structuring the metal absorber material by means of reactive ion etching in the submicron range.
A few years ago, an electron-beam projection process was proposed by S. D. Berger et al. In J.Vac.Sci.Technol. B9(6), November/December 1991, p. 2996-2999, "Projection electron-beam lithography: A new approach" which uses high-energy electrons and called for a new membrane mask technique. The SCALPEL.TM. mask (Scanning with Angular Limitation Projection Electron-Beam Lithography) also described by Huggins et al. in Proceedings of SPIE 1995, Vol. 2621, p. 247-255 and by J. A. Liddle et al. in Proceedings of SPIE 1994, Vol. 2322, p. 442-451 resembles the closed membrane masks used for X-ray beam lithography.
The layer thicknesses of the membrane and the metal absorber layer are smaller in the case of the SCALPEL masks. Electrons of approximately 100 keV penetrate both layers, but are scattered to a different extent in the different layers.
In contrast to the membrane masks used for X-ray beam lithography, the SCALPEL mask is divided into smaller mask fields. This division permits support walls, which provide a better mechanical and thermal stability. To keep the area loss between the mask fields to a minimum, the thin support walls are disposed perpendicular to the wafer surface and have been produced by anisotropic wet-etching from a (110) wafer.
Similarly to X-ray beam lithography masks, stress problems occur in the SCALPEL masks due to the membrane and/or the metal absorber layer. In the masks described inter alia in Proceedings of SPIE 1995, Vol. 2621, p. 247-255, the mask fields are long narrow strips, so that the unsupported membrane parts consist of rectangles of approximately 1 mm.times.2 cm in size. Since the membranes must be under tensile stress, different tensile stresses occur in the x-and y-direction, leading to an anisotropic distortion of the mask pattern.
U.S. Pat. No. 5,260,151 shows SCALPEL masks with square mask fields of approximately 1 mm edge length, in which the 0.1 mm thick and 1.0 mm high support walls delimiting the mask fields from one another are disposed perpendicular to the membrane of polycrystalline silicon. An isotropic stress distribution is thus achieved in the membrane. The manufacture of the thin, vertical support walls using anisotropic plasma etching techniques without damaging the membrane is admittedly problematic.